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Launch and Orbital Characteristics

Launch of Nimbus-7, carrying the SMMR sensor, was 25 October 1978 from Vandenberg Air Force Base, California. The Nimbus-7 platform was placed into a sun-synchronous orbit at an altitude of 955 km. The equatorial crossings are local noon for ascending node and local midnight for descending node. The spacecraft inclination is 99.1 degrees with a maximum poleward latitude of 80.77 degrees. The orbit period is 104.16 minutes. Equator crossings on consecutive orbits are separated by 26.1 degrees longitude.

Temporal and Spatial Coverage

The SMMR sensor operated 25 October 1978 through 20 August 1987. The sensor was placed in an alternate-day operating pattern on 19 November 1978 due to spacecraft power limitations. The SMMR provided complete global coverage every six days. Polar regi ons (poleward of 72 degrees) have complete coverage for each day the sensor was recording data. See sec. B.2.1 for a list of major data gaps during the period of record.

Sensor Characteristics

A parabolic antenna 79 cm in diameter reflected microwave emissions into a five-frequency feed horn. The antenna beam was at a constant nadir angle of 42 degrees, resulting in an incidence angle of 50.3 degrees at Earth's surface. The antenna was forward-viewing and rotated equally (+/- 25 degrees) about the satellite subtrack. The 50 degree scan provided a 780 km swath of the Earth's surface. Scan period was 4.096 seconds.

The SMMR sensor is a ten-channel device. The five dual-polarized (horizontal, vertical) frequencies range from 6.6 Gigahertz (GHz) to 37.0
GHz. See Gloersen and Barath (1977) for a complete description of the SMMR instrument. Please note that 21 GHz (channel 4) data are not included with this data set, because the drift in the channel is so great as to render the implemented correction for polarization mixing useless after the first two years.

Some words of caution regarding the effects of antenna patterns on mapping SMMR data are in order. The map grid size (about 25 km) is nominally half the size of the integrated field of view of the 1.7-cm channels. This, plus the fact that the size of the integrated field of view is determined from the half-power points on an approximately Gaussian-shaped antenna beam pattern, the integrate-and-dump time of the scan, and the spacecraft motion, results in high-radiance objects such as land or sea ice spi lling over onto areas of lower radiance such as open water. The land mask used in the SMMR maps does not take this into account, and therefore land signals will appear outside the land mask, giving false impressions of sea ice within 50-100 km of the coastal boundaries. In summer, when coastlines are ice-free, the land signals are high and give a misleading indication of ice. For example, the summertime minima in the Bering, and Okhotsk and Japan regions show this effect. This should be kept in mind when studying curvesof ice extent and area in Chapters 3 to 5 [of Gloersen et al., 1992]. In addition, the 50ø incidence angle for SMMR means that a large portion of the backward-looking wings of the SMMR antenna pattern intercepts the surface of the Earth. This gives rise to large differences in the SMMR response to ocean-land and ocean-ice boundaries during the northbound and southbound portions of the spacecraft orbit. These land-ocean and ice-
ocean spillover effects are most dramatically illustrated by the differences
in the daily northbound/southbound sea ice extents for the Antarctic
during austral winter. These differences are on average about 0.8 x 106
km2, or about 4% of the total extent (see Chap. 4 [of Gloersen et al.,
1992]). In the Arctic, the overall wintertime northbound/southbound
differences are about 0.2 x 106 km2, about one-fourth the values in the
Antarctic. This contrast results from the fact that the entire perimeter
of the Antarctic ice cover is bounded by open ocean whereas in the Arctic
much of the perimeter is bounded by land. The northbound/southbound
differences are absent in the images in [Chapters 3 to 5 of Gloersen et
al., 1992], since the data were averaged over the two portions of the orbits. (Gloersen et al. 1992, sec. 2.2.3, p. 22.)